Arctic Methane Claims Questioned

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A scientific controversy erupted this week over claims that
methane trapped beneath the Arctic Ocean could suddenly escape,
releasing huge quantities of methane, a greenhouse gas, in coming
decades, with a huge cost to the global economy.

The issue being debated is this: Could the Arctic seafloor really
fart out 50 billion tons of methane in the next few decades? In a
commentary published in the journal Nature on Wednesday (July
24), researchers predicted that the rapid shrinking of
Arctic sea ice would warm the Arctic Ocean, thawing
permafrost beneath the East Siberian Sea and releasing methane
gas trapped in the sediments. The big methane belch would come
with a
$60 trillion price tag, due to intensified global warming
from the added methane in the atmosphere, the authors said.

But climate scientists and experts on methane hydrates, the
compound that contains the methane, quickly shot down the
methane-release scenario.

"The paper says that their scenario is 'likely.' I strongly
disagree," said Gavin Schmidt, a climate scientist at the NASA
Goddard Institute for Space Studies in New York.

An unlikely scenario

One line of evidence Schmidt cites comes from ice core records,
which include two warm Arctic periods that occurred 8,000 and
125,000 years ago, he said. There is strong evidence that summer
sea ice was reduced during these periods, and so the
methane-release mechanism (reduced sea ice causes sea
floor warming and hydrate melting) could have happened then,
too. But there's no methane pulse in ice
cores from either warm period, Schmidt said. "It might be a
small thing that we can't detect, but if it was large enough to
have a big climate impact, we would see it," Schmidt told
LiveScience.

David Archer, a climate scientist at the University of Chicago,
said no one has yet proposed a mechanism to quickly release large
quantities of methane gas from seafloor sediments into the
atmosphere. "It has to be released within a few years to have
much impact on climate, but the mechanisms for release operate on
time scales of centuries and longer," Archer said in an email
interview.

Today (July 26), Peter Wadhams, a co-author of the Nature
commentary, defended the work against critics in an essay
posted online.

"The mechanism which is causing the observed mass of rising
methane plumes in the East Siberian Sea is itself unprecedented,
and the scientists who dismissed the idea of extensive methane
release in earlier research were simply not aware of the new
mechanism that is causing it," wrote Wadhams, an oceanographer at
the University of Cambridge in the United Kingdom.

"But once the ice disappears, as it has done, the temperature of
the water can rise significantly, and the heat content reaching
the seabed can melt the frozen sediments at a rate that was never
before possible," Wadhams added. "David Archer's 2010 comment
that 'so far no one has seen or proposed a mechanism to make that
(a catastrophic methane release) happen' was not informed by the
... mechanism described above. Carolyn Ruppel's review of 2011
equally does not reflect awareness of this new mechanism,"
Wadhams wrote.

But Ruppel, a methane
hydrate expert at the U.S. Geological Survey who authored a
review of research on gas hydrates in 2011, also called the
sudden-thawing scenario unrealistic.

Much of the Arctic's methane sits in
permafrost buried under hundreds of meters of seafloor
sediments, Ruppel said. The deposits formed on exposed ground
during the last Ice Age, when sea levels were lower. The rising
seas have been warming the deposits for millennia. Any added
warming will have to work down through the thick sediment cap.

Much of the modeling predictions in the Nature commentary were
based on recent discoveries of rising methane plumes in the East
Siberian Sea. However, those plumes may be from methane hydrates
or from microbes.

"Methane release in the Arctic from both marine and terrestrial
sources is expected to increase with warming climate, as
documented in numerous papers," Ruppel said. "Much of the methane
may actually be produced in the shallow sediments by
microbial processes and be completely unrelated to methane
hydrates."

However, there has yet to be a detectable change in
Arctic methane emissions in the atmosphere over the past two
decades, Ed Dlugokencky, a research scientist with the National
Oceanic and Atmospheric Administration's Earth System Research
Laboratory, said in an email interview.